I am currently using Exchange 2007 into three sites in three cities and two of the sites are connected to Internet sending and receiving emails via Edge transport servers.

Now I am planning to upgrade to exchange 2013 CU1. I don't find any documentation on how to establish mail flow between Exchange 2007 and Exchange 2013. Will it be automatic or do i need to create specific connectors between them?

It should be automatic and you don't have to do anything. Below is the checklist that you need to consider, however not all the steps but whatever you are planning...

Checklist: Upgrade from Exchange 2007

• Marked as answer by Simon_WuMicrosoft contingent staff, Moderator Tuesday, June 25, 2013 10:08 AM

So for the most part it's automatic.  When you start switching over your connectors you need to ensure mail is being forwarded to the new CAS servers and that the connectors have the new CAS servers listed.

I have a similar situation and am trying to figure out why mail isn't flowing.  Right now we have the old 2007 server which is still in use and 4 new 2013 servers (2xCAS, 2xMailbox).  Ideally before i start switching any connectors id like to get all the mailboxes moved from 2007 to 2013 then move the connectors.

Before I do that though I have created a mailbox in my 2013 DAG and am trying to send email to it via the 2007 server.

Right now, if the destination mailbox is on the 2013 server, I want mail to flow from the internet, into the 2007 server, then over to the 2013 server.  Is this even achievable?

Was this question answered.  We're in the same situation now as we're upgrading to Exchange 2013 from 2007. The latest CU certainly helped.   Initially the test mailboxes on Exchange 2013 couldn't email each other - This was resolved with CU7 and using "Custom Settings" - manually entered IPs for DNS in the Exchange  Admin Center "DNS Lookups".

Issue at the moment Test mailboxes on Exchange 2013 cannot email mailboxes on 2007 or visa-versa and mail from external sources queues on the 2007 box.  

Any assistance will be greatly appreciated.